1. Field of the Invention
The present invention relates to a relay viewfinder optical system, and more particularly to a Kepler-type real-image viewfinder optical system.
2. Description of the Prior Art
Conventional relay viewfinder optical systems used in single-lens-reflex cameras generally include a number of lens elements, and are therefore inconveniently large, heavy, and expensive. These shortcomings can be overcome by reducing the number of lens elements, but doing so makes it difficult to correct aberration properly. In particular, chromatic aberration is difficult to correct with a reduced number of lens elements, since its correction requires a combination of lens elements made of materials having different dispersion.
On the other hand, for optical systems other than relay viewfinder optical systems, various proposals have been made to correct chromatic aberration by the use of diffracting optical surfaces (for example, U.S. Pat. No. 5,257,133, U.S. Pat. No. 5,148,314, Japanese Laid-open Patent Application H5-157963, U.S. Pat. No. 5,044,706, and U.S. Pat. No. 5,013,133.
However, in relay viewfinder optical systems, correction of chromatic aberration cannot be achieved without using a number of diffracting optical surfaces. The use of a number of diffracting optical surfaces not only leads to deterioration of diffraction efficiency and thus to aggravation of flare, but also makes relay viewfinder optical systems more expensive.
There are some other optical systems that have conventionally been known as having a diffracting optical surface. For example, an optical system composed of single lenses having a diffracting optical surface is proposed in a thesis by G. J. Swanson and W. B. Veldkamp, titled "Infrared Applications of Diffractive Optical Elements", SPIE proceedings, vol. 885, paper 22 (1988). Moreover, U.S. Pat. No. 5,446,588 proposes viewfinder optical systems in which a diffracting surface is used to correct chromatic aberrations.
The above thesis by Swanson et al. teaches that, in an achromatic lens composed of two, i.e. positive and negative, lens elements, chromatic aberration can be corrected by providing a diffracting optical surface only on the positive lens element. Moreover, U.S. Pat. No. 5,446,588 teaches that the number of lens elements constituting a viewfinder optical system can be reduced if a diffracting optical surface is used to correct chromatic aberration. However, no proposal has been made to date as to effective arrangement of a diffracting optical surface in an optical system whose optical function is determined by the arrangement of an optical element having positive power and an optical element having negative power relative to each other (for example, a zoom lens system provided with a lens unit having positive power and a lens unit having negative power).
As one type of Kepler-type real-image viewfinder optical system having a zoom function, U.S. Pat. No. 5,225,927 proposes a zoom-type viewfinder optical system suitable for use with high zoom ratios.
In the viewfinder optical system proposed in U.S. Pat. No. 5,225,927, the objective lens is of a zoom type constituted of positive, negative, and positive lens units, and includes a number of doublet lenses each composed of a positive lens having a large Abbe number and a negative lens having a small Abbe number so that chromatic aberration is corrected properly over the entire zoom range. However, the use of doublet lenses composed of two types of lenses inevitably makes the lens system larger and heavier.
On the other hand, in the viewfinder optical system proposed in U.S. Pat. No. 5,446,588, a diffracting optical surface is provided only in the eyepiece lens, and the optical performance of the optical system is evaluated by evaluating only the optical performance of the eyepiece lens. Accordingly, even if the objective lens of this optical system is provided with a zoom function, it is not possible to eliminate the chromatic aberration occurring in the objective lens, and thus it is not possible to correct chromatic aberration properly over the entire zoom range. Moreover, since the diffracting optical surface is provided in the eyepiece lens, the pattern of the diffraction grating becomes conspicuous when the observer looks at the eyepiece lens with his eye positioned away from the viewfinder. This is visually undesirable.